The present invention relates to semiconductor optical sensing devices and, more particularly, to an improved casing for such devices.
According to the prior art, semiconductor optical sensing devices, such as charge coupled devices (CCD), include a ceramic casing in which a semiconductor optical sensor chip is sealed. Typical prior art semiconductor optical sensing devices are shown in FIGS. 9(a), 9(b) and 10(a), 10(b). Although the prior art will be explained hereinafter in connection with a CCD image sensor, the optical sensor may be any type of optical sensing device such as, for example, a photodiode or an infrared sensor.
Referring now to FIG. 9(b), the prior art ceramic casing includes a ceramic substrate 59, lead frames 55, and a ceramic outer frame 60. A semiconductor optical sensor chip 58 is bonded onto the ceramic substrate 59. The lead frames 55 serve also as lead pins. The ceramic outer frame 60 works as an insulating frame for fixing a protective transparent plate 57 over the opening of the casing.
A recess portion in the bottom of the ceramic substrate 59 receives the semiconductor optical sensor chip 58. The semiconductor optical chip 58 is die-bonded onto the bottom of the recess portion. The lead frames 55 are patterned corresponding to the internal terminals of the sensor chip 58. The lead frames 55 are fixed between the ceramic substrate 59 and the ceramic outer frame 60 with an adhesive or a glass of low melting point 61. Each internal terminal of the sensor chip 58 is connected to the corresponding lead frame 55 via a bonding wire 56.
Transparent plate 57 is fixed on the upper face of the ceramic outer frame 60 with low-melting-point glass or adhesive 61 such that the transparent plate 57 closes the open end of the ceramic casing and seals the sensor chip 58 in the ceramic casing. The internal space of the ceramic casing is empty.
In the sensing device structure described above, the sensor chip 58 detects the light transmitted through the transparent plate 57 and outputs signals indicative of the detected light through the lead frames 55.
Referring now to FIG. 10(b), a CCD image sensor is in a surface mounting-type ceramic casing. The internal space of the casing is filled with a transparent resin.
A plastic substrate 63 has a semiconductor optical sensor chip 58 bonded onto its central part. Metal wiring patterns 65, corresponding to the internal terminals of the sensor chip 58, are formed on the plastic substrate 63. The metal wiring patterns 65 extend to the vicinity of the area where the sensor chip 58 is bonded.
The end portions of the metal wiring patterns 65 on the side of the sensor chip 58 are bonded to the bonding pads of the sensor chip 58 via bonding wires 56. The metal wiring patterns 65 are connected, via through-holes bored in the peripheral portion of the plastic substrate 63, to mounting terminals 66 patterned on the back surface of the plastic substrate 63. An insulating outer frame 64 on the metal wiring patterns 65 is bonded to the plastic substrate 63 with adhesive 68 such that the insulating outer frame 64 surrounds the wire-bonding portions of the metal wiring patterns 65 and the sensor chip 58. The space surrounded by the insulating outer frame 64 is filled with transparent resin 69 such that the transparent resin 69 completely seals the sensor chip 58 and the bonding wires 56. A transparent plate 57 is fixed to the outer portion of frame 64 with transparent resin or adhesive 70.
The basic principle of the optical sensing device shown in FIGS. 10(a) and 10(b) is same with that of the optical sensing device shown in FIGS. 9(a) and 9(b). The optical sensing device of FIGS. 10(a) and 10(b) is filled with the transparent resin 69, while the internal space of the casing in the optical sensing device of FIGS. 9(a) and 9(b) is empty.
In the optical sensing device of FIGS. 9(a) and 9(b), the internal space of the ceramic casing should be filled with an inert gas, to prevent deterioration of the sealed sensor chip or the bonding wires. However, even when the internal space of the casing is filled with an inert gas, the optical sensing device will inevitably deteriorate, if the temperature and the humidity are not controlled precisely.
Further, leakage tests conducted after the inert gas injection increases the package costs.
In the surface-mounting-type casing of FIGS. 10(a) and 10(b), the transparent resin 69 protects the sensor chip 58 and seals the casing. However, the transparent resin 69 expands or contracts due to the temperature changes around and in the casing. The expansion and contraction of the transparent resin 69 cause separation of the transparent resin 69 and the casing and bubbles in the transparent resin 69. In some cases, the bonding wires 56 are broken by the expansion and contraction of the transparent resin 69.
Moreover, the expansion and contraction of the transparent resin 69 changes the distance between the upper face of the transparent plate 57 and the upper face of the sensor chip 58. This distance change causes change in the optical properties of the sensing device.